The revised application proposes a 5-year study examining the effects of a group-based parent training program as early intervention (secondary prevention) with 200 parents with preschool children who have mild/moderate developmental disabilities. The efficacy of the Incredible Years Parent Training series (IYPT; Webster-Stratton, 1984, 1994) with developmental disabilities modifications (IYPT-DD; McIntyre, 2008a, 2008b) will be compared with treatment as usual. The IYPT-DD is an intensive 12-week group-based intervention (2.5 hours per week) that uses group discussion and videotape vignettes to target 5 curricular areas: child play, praise, rewards, effective limit setting, and handling challenging behavior. The treatment as usual condition will include individual child and family services afforded to children with developmental disabilities in preschool and community settings. The IYPT series has been demonstrated to be efficacious for decreasing behavior problems in typically developing children (Webster-Stratton, 1984, 1994); however, this approach has not been fully investigated with families of young children with developmental disabilities, who are also at risk for significant problem behavior. The PI has successfully pilot tested this intervention with DD modifications with parents who have preschool-age children with developmental disabilities. Preliminary evidence from a funded R03 grant suggests feasibility, acceptability, and efficacy in terms of reducing child behavior problems and negative parent-child interactions (McIntyre, 2008a, 2008b; McIntyre & Phaneuf, 2007; Phaneuf & McIntyre, 2007). This proposed study will build on the initial pilot work and use a longitudinal design and randomized controlled trial to test the efficacy of IYPT-DD for (a) reducing child maladaptive behavior and increasing child adaptive behavior immediately posttreatment and at 6-month, 12-month, and 18-month follow- up assessments; (b) reducing inappropriate/negative parent-child interactions, increasing positive parenting practices, and increasing parental competence and self-efficacy immediately posttreatment and at 6-month, 12-month, and 18-month follow-up assessments; (c) increasing adaptation to school as measured by teacher assessments of children's early sociobehavioral adjustment in kindergarten; (d) determining if changes in child maladaptive and adaptive behavior are a function of changes in parenting behavior; and (e) determining if contextual influences, such as parenting stress, maternal depression, and partner support/relationship quality, moderate child outcomes. Advancements in multivariate data analysis, such as general growth mixture modeling, will be used to study the environmental factors associated with the formation of adaptive and maladaptive developmental trajectories, as well as to determine the efficacy of the parent intervention relevant to altering those trajectories. Economic analyses, including cost analysis, benefit-cost analysis, and cost- effectiveness analysis, will be used to determine the impact of the intervention relative to cost burden and determine the feasibility of uptake in community settings.